One conventional electronic ballast circuit is one that employs an inductive ballast, typically dropping about 80% of the line voltage across the ballast element. There are many applications in which such an inductive ballast is effective. However, for in particular, low voltage, high current electric discharge lamps, such as negative glow lamps, an inductive ballast is highly inefficient. For example, in the particular case of a 15 volt high current negative glow lamp, undesirably substantially all of the RMS line voltage would be dropped across the ballast element.
Accordingly, the use of an inductive ballast for this type of lamp for a low voltage high current discharge lamp results in poor lamp efficiency. Because of the relatively high currents required by these lamps, the corresponding Joule heating loss (I.sup.2 R, eddy current, hysteresis, etc.) is much higher than for a lamp operating at lower currents. In this regard, the higher current referred to would be in the range of 2-5 amps and the lower currents would be less than one amp. Accordingly, it is desirable because of these poor efficiencies associated with inductive ballasts to instead provide a more efficient ballast circuit, particularly for use with discharge lamps including DC glow discharge lamps.
Prior U.S. patents that describe the use of capacitive ballasts with or without rectifier circuits include U.S. Pat. No. 2,356,369 to Abernathy; U.S. Pat. No. 4,288,725 to Morton; U.S. Pat, No. 4,172,981 to Smith; U.S. Pat. No. 4,500,812 to Roche; and U.S. Pat. No. 3,787,751 to Farrow. Modifications to inductive ballasts have been carried out. For example, inductive ballasts can be designed so that resistive and magnetic power losses are minimized. However, to minimize both the ballast weight and system power losses, a capacitive ballast is preferred, particularly for negative glow, low pressure discharge lamps, a capacitive ballast in conjunction with a bridge rectifier has been employed in the prior art. In this regard, refer to, for example, the article "Capacitor Ballast for a Compact Fluorescent Lamp" by Watanabe, J. Light & Vis. Env., Vol. 7, No. 1, 1983, pp. 7-14. In this article, refer in particular to the circuit of FIG. 17 employing the combination of a bridge rectifier and capacitor ballast.
Reference is also made herein to FIG. 1 for an illustration of the use of a capacitor ballast in conjunction with a full-wave rectifier bridge for operating a low voltage, high current DC discharge lamp. More particularly, FIG. 1 illustrates the ballast element as capacitor C. The full . wave rectifier bridge is comprised of diodes D1-D4 interconnected in the normal bridge rectifier configuration. The input AC signal which typically is a 120 volt AC signal is coupled at the terminals 10. The terminals 10 connect in series with the capacitor C to the input of the full-wave rectifier bridge. The output of the full-wave rectifier bridge may be considered as coupling to the glow discharge lamp 12.
The glow discharge lamp 12 is comprised of an anode 14 and a cathode 16. Also illustrated in FIG. 1 is the switch 20. The switch 20 couples, in one position thereof, across the cathode 16. The operation of the switch 20 is well known and as the construction thereof forms no part of the present invention, it is not described in any great detail herein. It is efficient to state that the switch 20 is operable for lamp starting. Refer, for example, to similar starting switch configurations found in U.S. Pat. No. 2,356,369 or U.S. Pat. No. 4,288,725 previously referred to.
One of the drawbacks associated with the electronic ballast circuit of FIG. 1 is the characteristic of the circuit of operating with a single hot spot operation regime for the lamp cathode. This is illustrated by the arrows 18 in FIG. 1. In essence, the discharge current flows to the same point on the cathode, as illustrated by arrows 18, during each half cycle of the AC signal.